Wall Construction for a Boiler Arrangement

ABSTRACT

A wall construction for a boiler arrangement. The boiler arrangement is formed of at least a furnace and a separator. The furnace has a grid, a bottom part, and an upper part. The separator is arranged by conduits in flow communication with both the upper part and the bottom part of the furnace. The conduits, together with the separator, form an external circulation of bed material. The upper part of the furnace has four vertical walls, and the bottom part of the furnace has a height and four walls extending from the grid up to the vertical walls. The wall construction includes at least one hollow beam being attached to a wall of the bottom part of the furnace and extending substantially over the entire height of the bottom part, and the at least one hollow beam being in flow communication with the external circulation for returning bed material into the furnace.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a wall construction for a boilerarrangement. The present invention is especially applicable inconnection with, for example, lower inclined walls of fluidized bedboilers and circulating fluidized bed boilers.

An ordinary fluidized bed boiler arrangement of the prior art comprisesa furnace, to which fuel, bed material and combustion air areintroduced. When combusting the fuel, heat is generated, and both bottomash and flue gases are generated. The flue gases are taken to aseparator, which separates solid particles from the gases. The solidparticles are then returned back to the furnace.

Structurally, a circulating fluidized bed boiler (CFB) generallyincludes a furnace having a bottom, side walls and a roof, and at leastone particle separator connected in flow communication with the upperpart of the furnace. At least some walls of the bottom part of thefurnace are normally inclined, such that the cross section of thefurnace increases upwardly. The part of the furnace having the inclinedwalls may be called a converging bottom part. In practice, all of thewalls and the roof of the boiler and the separator comprise water orsteam tubes to collect heat from the furnace. The walls at theconverging bottom part of the furnace are normally covered withrefractory material that resists abrasion better than metallic, water orsteam tube walls. The bottom of the furnace is provided with a grid forintroducing combustion, or suspending, or fluidizing gas, called primaryair, into the furnace, and for removing ash and other debris from thefurnace. The side walls of the furnace are provided with means forintroducing fuel and means for introducing secondary air into thefurnace, as well as start-up burners. The furnace is also equipped withmeans for feeding inert bed material, which is normally sand, into thefurnace. Very often, the introduction means (for the fuel, the secondaryair, and the bed material) are positioned in the converging bottom partof the furnace.

The particle separator separates solid particles from the flue gas and asuspension of the solid particles entering the separator from the upperpart of the furnace. The flue gases are taken for further treatment fromthe separator, and separated solids are recycled back to the lower portof the furnace via a recycling conduit, including a sealing device, suchas a loopseal. The purpose of the loopseal is to prevent gas fromflowing from the furnace to the separator via the recycling conduit.This solids circulation is called external circulation. In addition tovertical upflow of the flue gas and the suspension of the solidparticles in the furnace, entering finally into the separator inlet,there is a vertical downflow of particles near and along the furnacewalls. This solids circulation is called internal circulation.

Very often, in connection with the internal or the external circulationof solid material, or both, at least one fluidized bed heat exchangechamber has been arranged to transfer heat from the bed of fluidizedparticulate solids to a heat transfer medium. Such a fluidized bed heatexchanger is sometimes arranged in the external circulation, so that thesolids leaving the solids separator are discharged into the heatexchange chamber on their way back to the furnace (see, for example,FIG. 1 (Prior Art)). This kind of a fluidized bed heat exchange chambertypically hangs from the separator at a distance from the furnace wall.The interior of the heat exchange chamber is provided with heat exchangemeans for transferring heat from the solid material to the heat transfermedium flowing inside the heat exchange means.

Lately, it has been suggested that a fluidized bed heat exchange chamberfor recovering heat from the circulating solids in the externalcirculation could also be arranged upon the furnace wall, i.e.,supported by the vertical furnace wall.

The solids entering the furnace from the external circulation, i.e.,directly from the separator, or via the fluidized bed heat exchanger,are normally introduced into the furnace via one or more openings in thelower part of the furnace, i.e., through the inclined walls of theconverging bottom part of the furnace. The conduit taking the solidsback to the furnace is conventionally an independently suspended channelbetween the separator and the furnace. This kind of solids returnconduit does not run along the outer surface of the furnace, but slopesfrom the bottom of the separator or of the fluidized bed heat exchangechamber towards the converging bottom part of the furnace, occupying aconsiderable space between the separator/fluidized bed heat exchangerand the lower part of the furnace. As another alternative, i.e., whenthe fluidized bed heat exchange chamber is upon the furnace wall, thesolids discharged from the chamber are taken down to the grid area ofthe furnace, outside the furnace and introduced only there into thefurnace. This means that the return conduit runs down to the grid areain the vicinity of the furnace wall. This is a controversial feature,as, on the one hand, this kind of a structure leaves the space furtheroutside the furnace wall intact, i.e., makes the structure more compact,but, on the other hand, occupies its space directly from the surface ofthe furnace wall, preventing any structures from being positioned on orthrough the furnace wall.

The structures of a fluidized bed boiler not only need to withstand theload caused by the combustion, i.e., heat, which is, by nature, acontinuous, normally non-changing load, but also, both sub- andsuper-atmospheric pressures that tend to bend or to flex the planarboiler walls to curved ones. Since the boiler walls are made of weldedwater or steam tubes provided with fins or membrane plates between thetubes, the wall structure is very weak against bending or flexing, andneeds specific reinforcement structures. Therefore, the walls areprovided with both vertical and horizontal reinforcements or stiffeners,called buckstays. Normally, vertical buckstays are welded on the boilerwall so that their thermal expansion is the same as that of the boilerwall. Horizontal buckstays are arranged outside the vertical ones, andare arranged in slidable connection with the vertical buckstays, suchthat their thermal expansion may be different from (that is, less than)the boiler wall and the vertical buckstays.

Reinforcing the walls of the boiler furnace with horizontal and verticalbuckstays does not, in general, pose a problem. There is plenty of roomfor all reinforcements used for stiffening the vertical walls in theupper part of the furnace, as there are not that many other pieces ofequipment arranged on or through the furnace wall. However, the lowerpart of the furnace and, specifically, the converging bottom part of thefurnace, is provided with so many conduits and accessories thatpositioning both the conduits and accessories, as well as thereinforcements, in an optimal manner is difficult, and sometimes, inpractice, impossible. The bottom part of the furnace has to be providedwith at least the following conduits: fuel feed, bed material feed,primary air feed (in addition to the grid), secondary air feed,returning bed material feed (both from the separator, from the fluidizedbed heat exchange chamber in connection with the separator, and from thefluidized bed heat exchange upper chamber on the wall of the furnace),and a connection for at least one, and, more often, several, start-upburners. Additionally, there may be one or more fluidized bed heatexchangers positioned outside the converging walls of the bottom part ofthe furnace. And, further, the converging bottom part of the furnace isalso provided with service access doors. And, finally, since the numberof the above-listed conduits, heat exchangers and openings is normallymore than one, it is easy to see that finding an optimal location foreach component is such a challenging task that most often severalcompromises have to be made.

One way to optimize the structures available on the furnace wall is (asis already known from the prior art, and illustrated in a very schematicmanner in FIG. 2) to attach some parts of the conduit, returning bedmaterial to the bottom part of the furnace, to the wall of the bottompart of the furnace. The prior art return conduit receives circulatingbed material from an ordinary separator arranged at a distance from theboiler. The return channel originating from the separator is connectedto the top of the return conduit by means of a bellows arrangement thatallows some movement due, for instance, to changing temperatures. Thereturn conduit is formed of water/steam tube panels, such that it hastwo side walls and a back wall. One long vertical edge of a side wall iswelded to the back wall, and the other opposite edge to the outersurface of the furnace wall. The return conduit has, at its lower end, abottom wall that slopes from the lower end of the back wall towards anopening in the lower part of the inclined furnace wall for allowing thereturning bed material to flow from the return conduit into the furnace.The side walls have extensions extending from the bottom wall down tothe level of the grid, whereby, the side walls extend the full height ofthe inclined furnace wall. The water/steam tube panels forming the sidewalls are welded on both the vertical and inclined walls of the furnace,such that the upper end of the panels extends at a distance above thetransition between the inclined wall and the vertical wall. The backwall of the return conduit, on its part, extends from the upper ends ofthe side wall panels in parallel with the vertical furnace wall,downwards to the side of the inclined wall, such that the back wall ofthe return conduit terminates at a distance of, e.g., about one third ofthe height of the inclined wall below the transition.

The above-described structure utilizes the side walls of the returnconduit as vertical buckstays, i.e., they reinforce the inclined bottomwall of the furnace against pressure fluctuations in the furnace.However, the use of a water/steam tube panel as a reinforcing structurein the manner discussed above has a few disadvantages. First, since, forthe most part of the height of the inclined walls, the side walls alonecarry the load, i.e., the load carrying members are planar, practically,two-dimensional, and at right angles to the surface they are supposed toreinforce, the construction of the side walls requires special attentiondue to the local tendency of buckling of a planar reinforcement. Second,to be able to carry the load, the planar walls have to extend rather farfrom the inclined wall occupying some space, which, at least, could beused more effectively. Third, for a substantial part of the side wallheight, i.e., for the height below the bottom wall of the returnconduit, the side walls are not in contact with hot returning bedmaterial, whereby, the medium circulating in the wall panels can onlydissipate the heat that it has been able to recover from the hot bedmaterial in the upper portion of the conduit.

SUMMARY OF THE INVENTION

An object of the present invention is to find at least one solution toat least one of the problems discussed above.

Another object of the present invention is to reconsider the use ofwater/steam tube panels as reinforcing structures in view of the risk oflocal buckling of planar stiffeners.

Yet another object of the present invention is to reconsider the use ofload carrying structures as heat transfer surfaces in view of improvingtheir overall efficiency.

And a still further object of the present invention is to optimize theuse of space in the vicinity of the inclined wall of the boiler bottompart.

The above, and other objects of the present invention, are met with thewall construction for a boiler arrangement of this invention, the boilerarrangement being formed of at least a furnace and a separator, thefurnace having a grid, a bottom part, and an upper part, the separatorbeing arranged by means of conduits in flow communication with both theupper part and the bottom part of the furnace, the conduits togetherwith the separator forming an external circulation of bed material, theupper part of the furnace having four vertical walls, and the bottompart of the furnace having a height, the wall construction comprising atleast one hollow beam extending substantially over the entire height ofthe bottom part, the at least one hollow beam being attached to a wallof the bottom part, and the at least one hollow beam being in flowcommunication with the external circulation for returning bed materialinto the furnace.

Other features of the wall construction of the present invention can bedetermined from the appended claims.

By means of the wall construction of the present invention, at least thefollowing advantages over the prior art have been achieved:

-   -   The bed material return conduit can be used as a beam        reinforcing the lower inclined walls of a boiler furnace.    -   The heat recovery surface used as a reinforcing structure is        also used for recovering heat from the returning bed material        far more efficiently than before.    -   The extension of both the return conduit and the reinforcing        structures is substantially reduced when compared to prior art        structures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the wall construction of the present invention will beexplained in more detail with reference to the following drawings.

FIG. 1 is a schematic representation of a circulating fluidized bedboiler arrangement of the prior art.

FIG. 2 is a schematic vertical cross-sectional representation of a priorart furnace having a bed material return conduit attached on the outsidewall of the furnace.

FIG. 3 is a schematic vertical cross-sectional representation of a firstpreferred embodiment of the present invention.

FIG. 4 is a more detailed cross-sectional view of the first preferredembodiment of the present invention.

FIG. 5 is a schematic representation of a second preferred embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates a circulating fluidized bed boiler 10of the prior art. The boiler 10 comprises a furnace 12 with an upperpart having four substantially vertical side walls 32, a bottom parthaving four side walls, of which two are normally inwardly inclined sidewalls 34, a discharge conduit 14 in the upper part or upper end of thefurnace 12 for taking the flue gas and solid particles suspended therebyto a solids separator 16, a passage 18 arranged in the upper end of thesolids separator 16 for the removal of cleaned exhaust gas from thesolids separator 16, a recirculation conduit 20 at the lower end of thesolids separator 16 for returning at least a portion the separatedsolids, i.e., mostly circulating bed material, back to the bottom partof the furnace 12, a fuel feed 22 arranged at a lower side wall 34 ofthe furnace 12, and means 24 and 26 for introducing primary air andsecondary air, respectively, arranged at the bottom part of the furnace12. The fuel feed 22 may include a screw feeder, a drop leg, or apneumatic feeder, just to name a few alternatives. The primary air 24 isthe primary combustion gas that is also used to fluidize the bedmaterial, and is thus fed into the furnace 12 through the grid 36arranged at the bottom of the furnace 12. The secondary air 26 isintroduced into the furnace 12 through the lower side wall 34 thereofslightly above the grid 36.

A gas lock 28 has been arranged in the return conduit 20 for preventinggas from flowing from the furnace 12 via the return conduit 21 into thesolids separator 16. Here, the return conduit 20 is further providedwith a fluidized bed heat exchange chamber 30 for collecting heat fromthe recirculating solids to a heat transfer medium. The path of therecirculating solids/bed material is called the external circulation,and includes the separator 16 and all conduits and equipment between theupper part of the furnace 12 and the bottom part of the furnace 12 usedfor returning the bed material back to the furnace 12. The upper andlower side walls, 32 and 34, respectively, of the boiler 10, as well asthe ones of the solids separator 16 usually comprise water or steamtubes, or are made of water/steam tube panels, so that the water orsteam acts as the heat transfer medium. The fluidized bed heat exchangechamber may, in accordance with recent suggestions, be arranged on theoutside wall of the furnace 12, too, whereby the recirculation conduit20 or return leg would be running down to the grid area closer to thefurnace wall than in prior art arrangements.

It is known from the prior art, for example, as illustrated in a veryschematic manner in FIG. 2, to attach some parts of the conduit 60returning bed material to the bottom part of the furnace 12 to the wallof the bottom part of the furnace 12. The prior art conduit 60 receivescirculating bed material from an ordinary separator 16 arranged at adistance from the boiler 10. The return channel originating from theseparator 16 is connected to the top of the return conduit 60 by meansof a bellows arrangement that allows some movement due to, for instance,changing temperatures. The return conduit 60 is formed of water/steamtube panels, such that it has two side walls 62 and a back wall 64. Onelongitudinal vertical edge of a side wall 62 is welded to the back wall64, and the other opposite longitudinal edge to the outer surface of thefurnace wall 32/34. The return conduit 60 has further a bottom wall 66that slopes from the lower end of the back wall 64 towards opening 68 inthe lower part of the inclined furnace wall 34. The side walls 62 of theconduit 60 have extensions 62′ extending from the bottom wall 66 down tothe level of the grid 36, whereby the side walls 62, 62′ extend the fullheight of the inclined furnace wall 34, i.e., the full height of thebottom part of the furnace 12. The water/steam tube panels forming theside walls 62, 62′ are welded on both the vertical 32 and inclined 34walls of the furnace 12, such that the upper end of the panels extendsat a distance above the transition between the inclined wall 34 and thevertical wall 32. The back wall 64, on its part, extends from the upperends of the side wall panels in parallel to the vertical furnace wall32, downwards to the side of the inclined wall 34, such that the backwall 64 of the return conduit 60 terminates at a distance of, e.g.,about one third of the height of the inclined wall 34 below thetransition. The side walls 62, 62′ of the return conduit 60 are utilizedas vertical buckstays, i.e., they reinforce the inclined bottom wall 34of the furnace 12 against sub- and super-atmospheric pressures in thefurnace 12.

However, the use of a water/steam tube panel as a reinforcing structurein the manner discussed above has a few disadvantages. First, since, forthe most part of the height of the inclined wall, or that of the bottompart of the furnace 12, the side walls alone carry the load, i.e., theload carrying members are planar, practically, two-dimensional, and atright angles to the surface that they are supposed to reinforce, theconstruction of the side walls requires special attention due to thelocal buckling tendency of planar reinforcements. Second, to be able tocarry the load, i.e., to be strong enough, the planar walls have toextend rather far from the inclined wall. This both adds to the risk ofbuckling, and occupies some space that at least could be used moreeffectively. Third, for a substantial part of the side wall height,i.e., for the height below the bottom wall of the return conduit, theside walls are not in contact with hot returning bed material, wherebythe medium circulating in the wall panels can only dissipate heat thatit has been able to recover from the hot bed material in the upperportion of the conduit.

FIG. 3 illustrates, as a first preferred embodiment of the presentinvention, means to overcome at least some of the disadvantages of theprior art, which have been discussed above. In connection with FIG. 3, anovel structure of arranging a return conduit 70 in connection with theinclined wall 34 of the bottom part of a furnace 12 is discussed. Whencompared to the prior art return conduit 60, it is easy to see that thereturn conduit 70 of the present invention solves at least some problemsof the prior art. The return conduit 70 forms a three-dimensional beamthat extends, in this embodiment of the present invention, over theentire height of, and along the inclined lower wall 34 of the furnace12, and thus, forms a vertical buckstay reinforcing the inclined wall34. The beam/return conduit 70 is advantageously formed of two sidewalls 72 and a back wall 74, all made of water/steam tube panels. Theside walls 72 have two opposite longitudinal edges each, and so does theback wall 74. Each side wall is attached, preferably, by welding, bymeans of its longitudinal edge to the longitudinal edge of the back wallto form a U-shaped beam (the cross section thereof being shown in moredetail in FIG. 4). The beam is welded to the inclined wall 34 of thebottom part of the furnace along the other longitudinal edges of theside walls 72, such that a box-like hollow rectangular beam 70 isformed, the back wall 74 of the beam being parallel with the inclinedwall 34 of the furnace. In the vicinity of its lower end, the beam 70 isprovided with a sloping bottom wall 76 that guides the returning bedmaterial through an opening 68 in the inclined wall 34 to the grid areaof the furnace 12. In this embodiment of the present invention, thebottom wall has been arranged within the walls of the beam 70 such thatthe water/steam tube walls of the beam 70 extend substantially to thelevel of the grid 36, to which the front wall of the beam 70, i.e., theinclined wall, is attached.

Now that the hollow beam 70 extends the entire height of the bottom partof the furnace 12, or of the inclined wall 34, the beam 70, includingits back wall 74, is able to carry the pressure load of the furnace 12.Due to the back wall 75 carrying a substantial part of the pressureload, the horizontal extension (i.e., the extension perpendicular to theinclined side wall 34 of the furnace 12) of the side walls 72 may bemade significantly smaller than the side walls 62, 62′ in the prior art.This saves some space in the vicinity of the furnace 12. Also, partiallydue to the smaller horizontal extension, a significantly larger part ofthe inner surfaces of the side walls 72 is in heat transfercommunication with the returning bed material, whereby, the heatrecovery properties of the side walls 72 are significantly better thanthose of the side walls 62, 62′ of the prior art. Further, due to thebox-like structure of the reinforcements, the risk of local buckling ofthe side walls has, in practice, disappeared.

In accordance with the embodiment illustrated in FIG. 3, the hollow beam70 extends not only along the lower inclined wall 34 of the furnace 12,but also, along the upper vertical wall 32 of the furnace 12. Aprerequisite for this structure is that the vertical upper wall 32 isnot provided with horizontal buckstays at its lower part, but thehorizontal supporting has been performed some other way. It is alsopossible, however, in accordance with another preferred embodiment ofthe present invention (illustrated later on in FIG. 5), that the beam 70runs along the inclined lower wall 34 up to the upper end of theinclined wall, and then, bends outwardly to pass the lowermosthorizontal buckstay on the vertical furnace wall 32 or a secondary airheader, if such is arranged to replace the horizontal buckstay.

FIG. 4 illustrates a horizontal cross section of the inclined wall 34 ofthe bottom part of the furnace 12, and two hollow beams 70 attachedthereon. As shown in FIG. 4, the beam is formed of a back wall 74 andtwo side walls 72. The side walls 72 are welded on the outside surfaceof the inclined wall 34, so that the furnace wall 34 forms the fourth,i.e., front wall of the beam 70. Naturally, the cross section of thebeam/return conduit 70 need not be always rectangular, but also, othershapes are applicable. In view of the present invention, it is importantthat both side walls 72 and the back wall 74 of the beam 70 participatein carrying the load subjected to the inclined wall 34 of the furnace12. As another alternative, the beam 70 may also be prefabricated offour water/steam tube panels, to a full box-like structure, such thatwhen installing and attaching the beam on the inclined wall, one of thebeam walls (the front wall) is positioned against the furnace wall,whereby, there are two walls between the return conduit and the furnacecavity. In this alternative, naturally, the additional front wall of thebeam carries some load, too.

FIG. 5 schematically illustrates the beams/return conduits 70 of aninclined lower wall 34 of the furnace 12 in accordance with a secondpreferred embodiment of the present invention. Here, it is assumed thaton the upper portion of the vertical furnace wall 32, there are threefluidized bed heat exchange chambers, from which the returning bedmaterial is taken down to the bed area of the furnace, by means of sixreturn conduits (two for each fluidized heat exchanger) that have beenpositioned substantially evenly along the horizontal width of theinclined lower wall 32 of the furnace 12. The overall construction ofthe beams is similar to the one already discussed above. However, thelower ends of the beam are somewhat different from the one shown in FIG.3, as here, the lower ends of the side walls terminate at the bottomwall of the return conduit/beam, the bottom wall sloping towards thegrid, as has been discussed earlier. This kind of beam construction ispossible, as the forces subjected by the inclined wall to the beam arevery limited in the vicinity of the grid, as the inclined wall issupported at its lower end by the grid, and is thus not able to bend asmuch farther away above the grid level. Thus, it could be said that thefull three-dimensional beam having preferably, but not necessarily,fixed width side walls, should extend over at least 80%, preferably, atleast 90% of the inclined wall height. For the rest of the beam length,the beam cross-sectional area, or the width of the side walls, could bedecreasing towards the lower end of the inclined wall eithercontinuously or discontinuously.

The bottom wall of the return conduit/beam is preferably made of awater/steam tube panel. In other words, as a preferred option, thebottom wall could be manufactured of the same panel as that of the backwall, just by bending the panel sufficiently. Another thing worthmentioning, in view of the embodiment shown in FIG. 5, is that the upperparts of the beams have been shown to bend outwardly, so that they leavea space between the furnace wall and the beam. In this variation of theinvention, the lower end of the vertical furnace wall has been providedwith a horizontal buckstay or a secondary air channel acting as abuckstay, which, on the one hand, is used as the upper attachment pointfor the vertical beams, but which, on the other hand, the beams have topass to be able to act as return conduits for the circulating bedmaterial. As to the vertical wall of the furnace and its stiffening, itis possible to use the return conduits or return legs as the verticalbuckstays, but the vertical wall may as well have vertical buckstays ofits own, as there is space enough for both buckstays and return legs.

The return conduits or beams of the present invention may be arranged inflow communication, in addition to a fluidized bed heat exchange chamberarranged on the upper wall of the boiler 10, with solids separator 16,with a fluidized bed heat exchange chamber hanging from the solidsseparator 16, and with a fluidized bed heat exchange chamber supportedseparate from the boiler 10 and the solids separator 16.

In view of the description above, it has to be understood that only afew most preferred embodiments of the present invention have beendiscussed. Thus, it is obvious that the invention is not limited to theabove disclosed embodiments only, but that it can be modified in manyways within the scope of the appended claims. It has to be understood,too, that features of a specific embodiment of the invention may beapplied in connection with features of other embodiments within thebasic idea of the present invention, or that the features from differentembodiments may be combined, as long as they result in a working andtechnically feasible construction.

Additionally, it is clear, as already discussed above, that one or morebeams/return conduits may be used to reinforce the wall of the bottompart of the furnace. The beams may form the sole vertical reinforcingmeans of the bottom part of the furnace, or they may be used togetherwith ordinary buckstays, such that at least one buckstay and at leastone return conduit/beam form the vertical reinforcement. In a similarmanner, the return conduits/beams may be used, in the above-describedmanner, to reinforce not only one inclined wall of the bottom part ofthe furnace, but also, the opposite inclined wall, and optionally, alsoat least one vertical wall of the furnace bottom part. And, finally, theinvention is applicable in reinforcing one or more side walls of thebottom part of the furnace irrespective of the inclination of the wall.That is, the wall/walls may be vertical or inclined in any direction.

1. A wall construction for a boiler arrangement, the boiler arrangementbeing formed of at least a furnace and a separator, the furnace having agrid, a bottom part, and an upper part, the separator being arranged byconduits in flow communication with both the upper part and the bottompart of the furnace, the conduits together with the separator forming anexternal circulation of bed material, the upper part of the furnacehaving four vertical walls, and the bottom part of the furnace having aheight and four walls extending from the grid up to the vertical walls,the wall construction comprising: at least one hollow beam beingattached to a wall of the bottom part of the furnace and extendingsubstantially over the entire height of the bottom part, and the atleast one hollow beam being in flow communication with the externalcirculation for returning bed material into the furnace.
 2. The wallconstruction according to claim 1, further comprising an opening in thewall of the bottom part of the furnace for receiving bed material fromthe external circulation, the opening being arranged in flowcommunication with the at least one hollow beam.
 3. The wallconstruction according to claim 1, wherein the at least one hollow beamextends over at least 80% of the height of the wall of the bottom partof the furnace.
 4. The wall construction according to claim 1, whereinthe at least one hollow beam is attached, at its lower part, to the gridof the furnace.
 5. The wall construction according to claim 1, furthercomprising a lowermost horizontal reinforcement at the lower end of thevertical wall of the upper part of the furnace, wherein the at least onehollow beam has an upper part, and the beam is attached, at its upperpart, to the lowermost horizontal reinforcement.
 6. The wallconstruction according to claim 1, wherein the at least one hollow beamis arranged in flow communication with one of (i) a fluidized bed heatexchange chamber arranged on the vertical wall of the upper part of theboiler, (ii) a fluidized bed heat exchange chamber hanging from theseparator, (iii) a fluidized bed heat exchange chamber supportedseparate from the boiler and the separator, and (iv) a separator.
 7. Thewall construction according to claim 1, wherein the at least one hollowbeam is formed of water/steam tube panels forming at least two sidewalls and one back wall, the panels being attached to each other and tothe wall of the bottom part of the furnace for forming a box-likestructure.
 8. The wall construction according to claim 7, wherein theback wall of the at least one hollow beam is parallel with the wall ofthe bottom part of the furnace.
 9. The wall construction according toclaim 7, wherein the at least one hollow beam has a lower part and abottom wall is arranged to the lower part of the beam.
 10. The wallconstruction according to claim 9, wherein the bottom wall is arrangedbetween the side walls of the at least one hollow beam.
 11. The wallconstruction according to claim 9, wherein the bottom wall is formed bybending the back wall of the at least one hollow beam between the sidewalls.
 12. The wall construction according to claim 9, wherein thebottom wall of the at least one hollow beam is arranged to slope towardsthe opening.
 13. The wall construction according to claim 1, wherein theat least one hollow beam is formed of two side walls and a back wall,each having two longitudinal edges, the side walls having a longitudinaledge and being attached by their one longitudinal edge to the wall ofthe bottom part of the furnace and by an opposite longitudinal edge tothe longitudinal edges of the back wall, to form a box-like structure.14. The wall construction according to claim 13, wherein the box-likestructure forms a hollow beam.
 15. The wall construction according toclaim 1, wherein the wall of the bottom part of the furnace has one ormore hollow beams acting as substantially vertical reinforcements orbuckstays.
 16. A boiler arrangement comprising: a furnace having a grid,a bottom part, and an upper part, the upper part of the furnace havingfour vertical walls, and the bottom part of the furnace having a heightand four walls extending from the grid up to the vertical walls; aseparator arranged by conduits in flow communication with both the upperpart and the bottom part of the furnace, the conduits together with theseparator forming an external circulation of bed material; and a wallconstruction comprising at least one hollow beam being attached to awall of the bottom part of the furnace and extending substantially overthe entire height of the bottom part, and the at least one hollow beambeing in flow communication with the external circulation for returningbed material into the furnace.
 17. The boiler arrangement according toclaim 16, further comprising an opening in the wall of the bottom partof the furnace for receiving bed material from the external circulation,the opening being arranged in flow communication with the at least onehollow beam.
 18. The boiler arrangement according to claim 16, whereinthe at least one hollow beam extends over at least 80% of the height ofthe wall of the bottom part of the furnace.
 19. The boiler arrangementaccording to claim 16, wherein the at least one hollow beam is attached,at its lower part, to the grid.
 20. The boiler arrangement according toclaim 16, further comprising a lowermost horizontal reinforcement at thelower end of the vertical wall of the upper part of the furnace, whereinthe at least one hollow beam has an upper part, and the beam isattached, at its upper part, to the lowermost horizontal reinforcement.21. The boiler arrangement according to claim 16, wherein the at leastone hollow beam is arranged in flow communication with one of (i) afluidized bed heat exchange chamber arranged on the vertical wall of theupper part of the boiler, (ii) a fluidized bed heat exchange chamberhanging from the separator, (iii) a fluidized bed heat exchange chambersupported separate from the boiler and the separator, and (iv) aseparator.
 22. The boiler arrangement according to claim 16, wherein theat least one hollow beam is formed of water/steam tube panels forming atleast two side walls and one back wall, the panels being attached toeach other and to the wall of the bottom part of the furnace for forminga box-like structure.
 23. The boiler arrangement according to claim 22,wherein the back wall of the at least one hollow beam is parallel withthe wall of the bottom part of the furnace.
 24. The boiler arrangementaccording to claim 22, wherein the at least one hollow beam has a lowerpart and a bottom wall is arranged to the lower part of the at least onehollow beam.
 25. The boiler arrangement according to claim 24, whereinthe bottom wall is arranged between the side walls of the at least onehollow beam.
 26. The boiler arrangement according to claim 24, whereinthe bottom wall is formed by bending the back wall of the at least onehollow beam between the side walls.
 27. The boiler arrangement accordingto claim 24, wherein the bottom wall of the at least one hollow beam isarranged to slope towards the opening.
 28. The boiler arrangementaccording to claim 16, wherein the at least one hollow beam is formed oftwo side walls and a back wall, each having two longitudinal edges, theside walls having a longitudinal edge and being attached by their onelongitudinal edge to the wall of the bottom part of the furnace and byan opposite longitudinal edge to the longitudinal edges of the backwall, to form a box-like structure.
 29. The boiler arrangement accordingto claim 28, wherein the box-like structure forms a hollow beam.
 30. Thewall construction as recited in claim 16, wherein the wall of the bottompart of the furnace has one or more hollow beams acting as substantiallyvertical reinforcements or buckstays.